This invention relates generally to a method for reducing the incidence and severity of explosions in processing equipment such as impact mills comminuting combustible materials.
More specifically, this invention relates to the use of a micron-sized dispersion of water droplets within an impact mill to reduce explosion hazards therein.
It is becoming increasingly common to process municipal solid waste to recover and recycle at least some of the components thereof or to produce a shredded product more acceptable for landfilling. Shredding municipal refuse in a hammer mill, chain mill or the like is a necessary first step in obtaining a relatively homogeneous waste stream in which the components thereof are physically liberated and reduced in size so as to be amenable to automated classification and separation techniques.
Municipal solid waste is a diverse mixture of materials and typically contains a major portion of paper and paper products with lesser quantities of glass, steel and aluminum cans and all other materials discarded into trash. The inclusion of flammable liquids, including alcohol, gasoline, paint and paint solvents and the like within the trash is not unusual. Explosive materials including ammunition, powder, commercial explosives and military ordnance are occasionally encountered. Impact shredding of this diverse mixture results in relatively frequent explosions within the shredder as ignition sources are readily available both from sparks produced by the hammers striking metal and from localized temperature buildups within the shredder. In fact, through 1975 it has been reported that explosions occurred with a frequency of one explosion for every 85,000 tons shredded. About 70% of those explosions caused significant property damage.
It is recognized and accepted in the art that explosions of materials containing their own oxidant, military ordnance and commercial explosives, for example, cannot be suppressed or significantly attenuated. However, explosions from dust and flammable vapor or liquid are amenable to suppression or attenuation. Two general approaches to explosion suppression within impact mills have been taken in the art. In the first approach, a sensing device detects an incipient explosion and releases a charge of fire or ignition suppressing chemicals or gases into the mill. While this system is well proven in many applications, it does not work well in a hammer mill environment. Because of the large number of explosive events occurring in a typical hammer mill, often one or more per day, the cost in recharging the first suppressing chemicals or gases becomes prohibitive. As a result, the suppression system is often not recharged leading to a complete lack of explosion control.
A second approach to explosion suppression or attenuation was reported in a paper entitled, "An Approach to Energy Attenuation of Explosive Wastes in Processing Equipment," published in the Proceedings of the Sixth Mineral Waste Utilization Symposium, Chicago, Ill., May 2-3, 1978. This paper reported that a high pressure water spray through nozzles to produce a droplet dispersion having a median particle size in the general range of about 30 to 45 microns in the mill environment tended to significantly reduce the severity of explosions.